Objective

• Allow apps to order their CursorIcon inserts strictly before they are applied/updated.

Solution

• Add a new system set.
• Open question: Should the set exist in bevy_window a la CursorIcon so that call sites don't depend on bevy_winit specifics for this ordering? In that way, bevy_winit would import the system set from bevy_window and itself become a "user" of the system set. One could imagine a future bevy_sdl backend would use the same system set from bevy_window. Might be fine as I've done it though?

Testing

• Ran the custom_cursor_image example.